1. Field of the Invention
The present invention relates to evaluation and adjustment methods of an optical receiver and an optical communication system, and in particular, to the evaluation and adjustment methods of an optical receiver and optical communication system for evaluating and adjusting a delay and a level difference in two optical paths between a delay interferometer and the optical receiver.
2. Description of the Related Art
An optical fiber communication system is an important technology for realizing long-distance and large-capacity communication. The optical fiber communication system which is currently commercialized uses an intensity modulation method. The intensity modulation method is a method wherein transmission is performed by assigning “1” or “0” of a digital signal to whether or not there is an optical pulse. The intensity modulation method has been applied to commercialization by taking advantage of being a method of easily generating and detecting a modulation signal and capable of long-distance transmission.
Optical fiber communication is required to be faster in conjunction with increasingly larger capacity of information transmission in recent years. While a currently commercialized data transfer rate is 10 Gbps at the maximum, research and development of a system of 40-Gbps data transfer rate as a next-generation system are committedly underway at present. Furthermore, cost reduction due to extended transmission distance is also strongly required so that a transmission technology for a distance exceeding 1,000 km is ongoing.
There are two major problems in realizing higher speed and longer distance of the optical fiber communication. Firstly, it is a countermeasure against optical noise accumulation. If a transmission rate is heightened in the intensity modulation method, signal bands to be used increase and so an amount of noise that the system receives also increases. As a result of this, a value of a signal-to-noise ratio becomes smaller at a receiving end so that code errors increase, which is quality deterioration.
In the case of rendering the transmission distance longer, it is necessary to increase optical amplifier repeaters for loss compensation. The signal-to-noise ratio also deteriorates at the receiving end due to accumulation of optical noise generated by the optical amplifiers. Therefore, it is necessary, for the sake of realizing the higher speed and longer distance, to reduce the optical noise accumulation or develop a transmission system which is strong against the optical noise accumulation.
In recent years, concerning such a problem of the optical noise accumulation, attention has been focused on a phase modulation method, and in particular, on a differential phase shift keying (DPSK) method. The DPSK method is a method of changing optical phases of adjacent bit slots by 180 degrees in order to transmit “1” and “0” of digital signals. Especially, attention is focused on a system combining the DPSK method with a 1-bit delay detection receiving system in terms of its high performance and easiness of configuration.
This system changes the phases of the bit slots by 180 degrees in the case of “1” and leaves the optical phases as-is in the case of “0” as to transmit data at a transmitting end. At the receiving end, it branches a received signal, places a 1-bit delay element on one branch and then causes the two signals to interfere. As a result of this, intensity of interference signals becomes maximal if the phases are the same as the signals of immediately preceding bit slot, and quenching occurs if a phase difference becomes 180 degrees. This principle is used to convert information applied to a phase change to intensity information and receive it.
Use of the DPSK method allows communication with few errors even in a receiving state in which the signal-to-noise ratio is lower than that of the intensity modulation method. A reason for this is indicated below. FIGS. 9A and 9B are diagrams showing an example of a relation of intersymbol distance between a DPSK signal and an ordinary intensity signal. FIG. 9A shows placement of “1” and “0” codes on a complex electric field plane by the intensity modulation method, and FIG. 9B shows the placement by the DPSK method.
According to FIGS. 9A and 9B, the distance between the “1” and “0” codes by the DPSK method is twice as large as that of the intensity modulation method. Because of such a placement relation, the DPSK method takes a twice more amount of noise, that is, half the signal-to-noise ratio to obtain the same code error rate as in the case of the intensity modulation method. Thus, the DPSK method is a transmission system which is strong against the noise and suited to higher speed and longer distance of the optical fiber communication.
A second problem is a countermeasure against distortion of an optical waveform. In the optical fiber communication, a main cause for distorting the optical waveform is a nonlinear optical effect of an optical fiber. In the case of the intensity modulation method, it is known that the distortion due to this effect becomes more significant as the transmission rate increases. As for the long-distance transmission, it is also known that accumulation of waveform distortion due to the nonlinear optical effect is a serious problem. Therefore, it is necessary, for the sake of realizing longer-distance and larger-capacity communication, to use an optical fiber of a low nonlinear optical effect or a transmission system which is strong against the nonlinear optical effect.
As for this problem, there is a proposed method of rendering each individual bit of the DPSK modulation signal as an RZ (Return to Zero) pulse and transmitting it (refer to the Patent Document 1 (Japanese Patent Laid-Open No. 2003-60580 for instance)). This method is called an RZ-DPSK method. The RZ-DPSK method suppresses the waveform distortion from two aspects by rendering a signal of each individual bit of the DPSK signal as the RZ pulse.
One is the effect that transmission at lower power is possible because light intensity of a peak portion can be higher than the same average light intensity and consequently the signal-to-noise ratio can be earned by rendering each individual bit as RZ. Another is the effect of suppressing pulse interference among the bits by implementing an RZ form. As a result of these, the method has been rapidly recognized as a method of allowing an ultralong distance transmission in 40-Gbps transmission in recent years.
The RZ-DPSK method uses a delay interferometer for converting a phase modulation signal to an intensity modulation signal in a receiver (for instance, refer to the Non-Patent Document 1 (“Demonstration of 42.7-Gb/s DPSK Receiver With 45 Photons/Bit Sensitivity”, “A. H. Gnauck, S. Chandrasekhar, J. Leuthold, L. Stulz, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 1, p. 99 to 101, January 2003”)). This method is called a delay interference detection method, which has an advantage that the optical signal for the local oscillator is no longer necessary and miniaturization is possible in comparison with a coherent receiving method.
FIG. 10 is a circuit diagram of an example of a Mach-Zehnder delay interferometer. With reference to FIG. 10, the delay interferometer is configured by including a directional coupler 301 for branching light of an input portion, a delay device 304 for providing one arm 302 after branching with a delay of a time slot equivalent to 1 bit or so of the bit rate of the phase modulation signal, the other arm 303, and a directional coupler 305 for multiplexing the light from the two arms again.
Phase modulation light inputted to the delay interferometer is halved into equal power by the directional coupler 301 to be led to the first arm 302 and the second arm 303. In this case, the light which passes a waveguide and gets coupled to the first arm 302 has its optical phase shifted by 90 degrees. The light having propagated through the two arms is branched in two by the output portion of the directional coupler 305 respectively.
A half of the light from the first arm 302 and a half of the light from the second arm 303 are outputted to a first output port 306 of the directional coupler 305. In this case, the light from the second arm 303 has its optical phase shifted by 90 degrees. As a result of this, both the light having passed through the first arm 302 and the light having passed through the second arm 303 have their phases shifted by 90 degrees.
A second output port 307 of the directional coupler 305 also has the half of the light from the first arm 302 and the half of the light from the second arm 303 outputted thereto. In this case, the light from the first arm 302 undergoes the phase shift of 90 degrees so that only the light having passed through the first arm undergoes the phase shift of 90 degrees plus 90 degrees, that is, 180 degrees.
As a result of this, in the case where the light from the first arm 302 and the light from the second arm 303 at the first output port 306 are put in the same phase by adjusting one of the arms, the light from the first arm 302 and the light from the second arm 303 have opposite phases at the second output port 307.
In this case, if CW (Continuous Wave) light is inputted to the interferometer, the light is mutually intensified and outputted at the first output port 306 while no light is outputted at the second output port 307 as a result of interference canceling due to the opposite phases.
A description will be given as to the operation when the DPSK signal enters the interferometer. The DPSK signal has the optical phases as “0”, “π” as against “0”, “1” of the digital signal. As a result of this, the first arm 306 of the interferometer has the light outputted in the same phase in the case where the adjacent bits have the same phase. In the case where the adjacent bits have different phases, that is, there is a difference of π, the light is mutually cancelled and quenched. As a result of this, phase information is transformed to intensity information.
The second arm 307 of the interferometer has the light quenched in the case where the continuous bits have the same phase. In the case where the bits have different phases, that is, there is a difference of π, the phase difference of the light between the two arms is 0 or 2π to be mutually intensified so that the light is consequently outputted.
As above, if the continuous bits of the DPSK signal have the same phase, the light is outputted from the first arm 306. If there is a difference of π, the light is outputted from the second arm 307.
As for the output of the interferometer, the port for outputting the light is switched due to the phase difference of the continuous bits. Therefore, an optical receiver performs differential reception for converting each of the two outputs of the interferometer to the electrical signal from the optical signal and calculating the difference of the signals. Thus, amplitude of the signal to be identified becomes maximal, and resistance to the aforementioned optical noise becomes highest.
To maximize the performance by the differential reception, it is created so that there is equal propagation delay from the directional coupler 305 of the output of the interferometer 107 shown in FIG. 10 to a differential calculation portion 110 of the optical receiver 108 including two photo-detectors 109A and 109B.
A related method shown in FIG. 11 has been used as a method of examining whether or not the propagation delay between the interferometer and the optical receiver is equal. FIG. 11 is a block diagram of an example of a related optical communication system. FIG. 11 shows an example of a configuration for measuring a delay and a level difference between the interferometer and the optical receiver.
With reference to FIG. 11, an optical signal to be used for the examination is generated by a DPSK optical transmitter portion 101. The light outgoing from a light source 102 has its phase modulated by a data modulator 103. The signal for driving data modulation is a signal from a random pattern generator 400. An output signal from the data modulator 103 is further modulated by a clock modulator 104 to convert the intensity to the pulse signal. The clock modulator 104 is driven by a clock signal 106.
An RZ-DPSK signal is generated by the above configuration. This optical signal is input to an interferometer 107. The optical signal input to the interferometer 107 is converted to the intensity information from the phase information and received by the differential optical receiver 108. One of the outputs of the interferometer 107 is input to the first photo-detector 109A, and the other output is input to the second photo-detector 109B so as to convert the outputs to the electrical signals from the optical signals. The signals converted to electrical signals are rendered as differential signals by the differential circuit 110 to be outputted. These output signals are monitored by a sampling oscilloscope 401 of which measurement band is wide enough for the received signals.
FIGS. 12A to 12F are diagrams showing an example of an eye pattern for measuring the delay and level difference between the related interferometer and optical receiver. FIG. 12A shows the case of monitoring with a wideband optical receiver having no delay difference and level difference. FIG. 12B shows the case of monitoring with the wideband optical receiver having no level difference with a delay difference of 20% of the bit slots. FIG. 12C shows the case of monitoring with the wideband optical receiver having no delay difference with a level difference of 1.5 dB. FIG. 12D shows the case of monitoring with the optical receiver having no delay difference and level difference, of which band is 70% of the bit rate. FIG. 12E shows the case of monitoring with the optical receiver having no level difference with a delay difference of 20% of the bit slots, of which band is 70% of the bit rate. FIG. 12F shows the case of monitoring with the optical receiver having no delay difference with a level difference of 1.5 dB, of which band is 70% of the bit rate.
In the case where the two paths have equal gain (loss) and propagation delay, a measured waveform becomes a waveform which is symmetrical side to side and up and down as in FIG. 12A. In the case where only the propagation delay is different between the two, the measured waveform when a propagation delay difference is 20% of bit slot width as an example is shown in FIG. 12B. In this case, the waveform becomes asymmetrical in a time axis direction.
FIG. 12C shows the measured waveform in the case where the two paths have unbalanced gain, that is, the case where a gain difference is 1.5 dB for instance. In this case, the waveform becomes asymmetrical in an amplitude direction (vertical axis direction). In the case where both the delay and gain are unbalanced, a monitored waveform becomes superposition of changes in these two-axis directions. The delay difference and gain difference between the interferometer 107 and the optical receiver 108 are examined by utilizing these graphic changes.
There is a disclosure of an example of the optical communication system using automatic feedback control for the sake of fine-tuning the delay interferometer (refer to the Patent Document 2 (Japanese Patent Laid-Open No. 2005-80304 (paragraphs 0009, 0024, 0026, 0045, FIGS. 1 and 5C) for instance)).
However, there is the following problem to the RZ-DPSK reception method using the interferometer indicated in the conventional technology.
The first problem is that the sampling oscilloscope 401 or the optical receiver 108 needs to be wideband, which is used to examine whether or not the delay and the gain are equal.
The reason for this is as indicated below.
As shown in FIGS. 12A to 12C, a waveform change in the output of the optical receiver 108 is utilized to examine whether or not the delay and gain are equal. Especially, examination accuracy is influenced by whether or not a rising edge and a trailing edge of the waveform and a pulse height can be accurately monitored since a time displacement and an amplitude difference of the pulse are judged from a graphic change.
As the influence exerted on the graphic change by the measurement band, the waveform was monitored in the case where the band of the sampling oscilloscope 401 is only 70% of the bit rate. In the case of FIG. 12D having neither delay difference nor gain difference, the waveform is symmetrical in both the time and amplitude directions.
In the case shown in FIG. 12E having the delay difference of 20% of the bit slots, the rising edge and trailing edge of the pulse are gentle in comparison with FIG. 12B so that an amount of displacement is rather unclear. Furthermore, in the case of FIG. 12F having the gain difference of 1.5 dB, deterioration of an asymmetry property is unclear in the amplitude direction due to lowering of a peak level of the pulse so that it is difficult to judge a difference from FIG. 12D.
Thus, the monitoring requires a measuring device of a wide band, which results in an expensive monitoring system. There is also a problem that the band of the monitoring system needs to be maintained.
The second problem is that an optimal state of the symmetric property and delay can only be graphically represented so that it becomes non-quantitative and an examination result becomes personal.
The reason for this is as indicated below.
As previously described, the related technology judges whether or not there are the delay difference and gain difference from the graphic changes of the waveform. Therefore, there is a possibility that a judgment of being alike or different as to a graphic characteristic of the waveform may depend on subjectivity of a measurer in addition to differences in a measurement environment and a device to be used. Definitive performance is quantifiable by using the code error rate and the like. In the case of making a judgment on the extent to which an output waveform graphic should be adjusted in order to optimize code error rate performance, however, it depends on experience and skills of a worker and a quantitative examination is difficult. As a result of this, there is a problem that it is difficult to perform a quantitative evaluation of the performance resulting from the delay difference and gain difference.
The delay that should be adjusted in the invention disclosed in the Patent Document 2 is optical length of an upper arm and a lower arm of a Mach-Zehnder interferometer (MZ1) (refer to paragraphs 0024 and 0026 of the Patent Document 2). In comparison, the two portions to be judged or adjusted in the present invention are the portions between the delay interferometer and the photo-detectors (PD), that is, the portions from the multiplexing by the delay interferometer onward. Therefore, the invention disclosed in the Patent Document 2 is entirely different from the present invention as to its object, configuration and action.